Because normal radio communications can be intercepted by anyone with a receiver, they lack the security required for transmission of classified information. During World War II, frequency hopping was used to make radio signals more difficult to detect and decode. In frequency hopping, the carrier frequency of a transmitter is rapidly switched among a large set of predetermined frequencies according to a sequence code that is known to the transmitter and must be known by a receiver to decode the communication. Although any radio can receive the frequency-hopped signal, only those radios that know the sequence code are able to decode the transmission. Conventional frequency-hopping radios use multiple fixed frequency voltage controlled oscillators and a switch or a fast-tuning voltage controlled oscillator to generate the different frequencies of the carrier signal. The size and power required by these voltage controlled oscillators can be limiting factors in producing compact power efficient radio devices.